1. Technical Field
The present invention relates to a porous glass preform production apparatus for producing a large glass preform used in the production of optical fibers (hereinafter, simply referred to as “optical fiber preform”), particularly by depositing glass microparticles generated by a flame hydrolysis reaction of the raw materials for glass in a burner flame. In the designated countries where incorporation of documents by reference is approved, the content described in the specification of the following patent application is incorporated into the present patent application by reference, and will be regarded as a part described in the present specification. Japanese Patent Application No. 2006-032686. Date of filing: Feb. 9, 2006
2. Related Art
The VAD method is a well known method for producing an optical fiber preform. In this method, a porous glass preform including a core layer and a cladding layer (hereinafter, simply referred to as a “porous preform”) is produced by fitting a starting member on a shaft supported by the upper structure of an apparatus, which the shaft elevates while rotating, so that the starting member is lowered into a reaction chamber, and depositing glass microparticles which have been generated by a core deposition burner and a cladding deposition burner installed in the reaction chamber, on the tip of the starting member.
The generated glass microparticles are not all deposited, and unattached glass microparticles which have not been deposited are generated throughout the production. Most of these unattached glass microparticles are discharged, together with other gases such as exhaust gas, from an exhaust outlet provided in the reaction chamber.
However, a portion of the glass microparticles attach to the ceiling or sidewalls of the reaction chamber, during the period from being generated in the burner flame to being discharged. There have been cases where these glass microparticles attached and deposited on the ceiling or sidewalls exfoliate and fall down to attach on a porous preform during the production, thereby causing the generation of air bubbles or foreign materials in the optical fiber preform after vitrification.
Recently, cost reduction has been demanded, and enlargement of optical fiber preforms has become an urgent issue. In order to increase the size of an optical fiber preform to be produced, the feed amount of raw materials must be increased. However, if the feed amount of raw materials is increased, the amount of excess glass microparticles which have not deposited increases, even though the deposition efficiency does not change. As a result, the frequency for the glass microparticles attached on the inner walls of the reaction chamber to exfoliate and fall down, also increases.
In order to solve such problems, Japanese Patent Application Publication No. 2002-193633 discloses a method for reducing glass microparticles, that is, soot, which attach to the ceiling, by providing a slit-shaped air supply inlet on the sidewall surface of the reaction chamber where a burner is installed, in the vicinity of the ceiling of the sidewall, and providing exhaust outlets on the sidewall surface which is opposite to this air supply inlet.
However, the method disclosed in JP 2002-193633 cannot avoid the attachment of soot to the ceiling which comes in contact with the back of the porous preform as viewed from the air supply inlet side, and thus it is difficult to solve the problem of exfoliation of deposited soot. Therefore, when attempting to avoid the attachment of soot by increasing the exhaustion efficiency and thereby increasing the amount of supplied exhaust, there occur new problems such as that the airflow in the chamber is disturbed, and the flame of the core deposition burner is disturbed, so that the deposition does not occur stably. There are also problems that striae are generated in the resulting optical fiber preform, or the optical properties in the length direction fluctuate.